US6642746B2

US6642746B2 - Phase detector with minimized phase detection error

Info

Publication number: US6642746B2
Application number: US10/247,878
Authority: US
Inventors: Kevin S. Donnelly; Thomas H. Lee; Tsyr-Chyang Ho
Original assignee: Rambus Inc
Current assignee: Rambus Inc
Priority date: 1996-01-02
Filing date: 2002-09-20
Publication date: 2003-11-04
Anticipated expiration: 2016-01-02
Also published as: US20030016057A1

Abstract

A phase detector is described that includes a load circuit that presents both a high differential impedance and a low common mode impedance. The load circuit is coupled to (1) a power supply and (2) a first node and a second node. The first and second nodes form an output of the phase detector. A capacitive circuit has (1) a first capacitor coupled to the first node and ground and (2) a second capacitor coupled to the second node and ground. A first circuit is coupled to the first and second nodes for detecting a phase difference between a first signal and a second signal. A second circuit is coupled to the first and second nodes for detecting the phase difference between the first and second signals and for minimizing phase detection error of the first circuit such that the phase difference between the first and second signals can be detected with minimized phase detection error. Each of the first and second circuits receives the first and second signals and a reference signal. The second circuit is cross-coupled to the first circuit such that an error current generated by the second circuit cancels that generated by the first circuit such that the phase detector detects the phase difference between the first and second signals with minimized phase detection error.

Description

RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 08/582,045, filed Jan. 2, 1996, which is now U.S. Pat. No. 6,340,900 and U.S. patent application Ser. No. 09/707,491, filed Nov. 6, 2000, which is now U.S. Pat. No. 6,480,035.

FIELD OF THE INVENTION

The present invention pertains to the field of phase detection circuits. More particularly, this invention relates to a phase detector that relatively accurately detects the phase difference between two input signals having different voltage swing characteristics, wherein the phase detector substantially minimizes the phase detection error arising from parasitic capacitance.

BACKGROUND OF THE INVENTION

Phase detection circuits are typically used in computer systems and other electronic systems for detecting the phase difference between two input signals. For example, in a phase locked loop (“PLL”) system, it is frequently desirable to generate a periodic signal waveform that is in a quadrature (i.e., 90 degree) phase relationship with a reference signal. This typically requires a phase detection circuit in the PLL system to detect any departure from the desired quadrature phase relationship between the two signals. The detected departure from the desired phase relationship between the two signals is typically referred to as quadrature phase error.

When the phase detection circuit detects the quadrature phase error, the amount of the quadrature phase error is then supplied to other circuits of the PLL system. These other circuits of the PLL system then compensate for the quadrature phase error of the two signals such that the desired quadrature phase relationship between the two signals is maintained. As is known, a PLL system is typically used to maintain stable frequency and phase characteristics of an input signal.

One type of prior art phase detection circuit for detecting the quadrature phase error of two signals is an exclusive-OR gate logic circuit. The exclusive-OR gate logic circuit detects the quadrature phase error by causing its average output voltage to be proportional to the quadrature phase error.

Disadvantages are, however, associated with the prior art exclusive-OR gate type quadrature phase detector. One disadvantage is that the prior art exclusive-OR gate type quadrature phase detector typically requires that its input signals have substantially similar voltage swing characteristics. If the input signals have different voltage swings, the average output voltage of the exclusive-OR gate typically cannot properly reflect the quadrature phase error detected.

Another disadvantage associated with such a prior art detector is that the prior art detector typically cannot accurately detect the quadrature phase error. This is often due to the fact that phase detection errors typically occur in the circuit during phase detection. One contributor to the phase detection errors is the parasitic capacitance in the circuit. Because of the unpredictable nature of the parasitic capacitance, it is often relatively difficult to compensate for the phase detection errors that arise from the parasitic capacitance.

SUMMARY AND OBJECTS OF THE INVENTION

One of the objects of the present invention is to provide a phase detector that accurately detects the phase difference between two input signals having different voltage swing characteristics.

Another object of the present invention is to provide a phase detector that substantially minimizes the phase detection error induced by the parasitic capacitance.

Another object of the present invention is to provide a phase detector that minimizes the phase detection error of the circuit by compensating for the parasitic capacitance that causes the phase detection error.

A further object of the present invention is to provide a phase detector for detecting the phase difference between a full voltage swing periodic signal and a low voltage swing quasi-differential or fully differential periodic signal, and for providing compensation for phase detection errors arising from parasitic capacitance such that substantially accurate measurement of the phase difference of the two signals can be obtained.

A phase detector is described that includes a load circuit that presents a high differential impedance and a low common mode impedance. The load circuit is coupled to (1) a power supply and (2) a first node and a second node. The first and second nodes form an output of the phase detector. A capacitive circuit has (1) a first capacitor coupled to the first node and ground and (2) a second capacitor coupled to the second node and ground. A first circuit is coupled to (1) the first and second nodes and (2) ground for detecting a phase difference between a first input signal and a second input signal. A second circuit is coupled to (1) the first and second nodes and (2) ground for detecting the phase difference between the first and second input signals and for minimizing phase detection error of the first circuit such that the phase difference between the first and second input signals can be detected with minimized phase detection error. Each of the first and second circuits receives the first and second input signals and a reference signal. The first and second circuits are cross-coupled such that an error current generated by the second circuit cancels that generated by the first circuit so that the phase detector detects the phase difference between the first and second signals with minimized phase detection error.

A phase detector is described that includes a load circuit that presents both a high differential impedance and a low common mode impedance. The load circuit is coupled to (1) a power supply and (2) a first node and a second node. The first and second nodes form an output of the phase detector. A capacitive circuit includes (1) a first capacitor coupled to the first node and ground and (2) a second capacitor coupled to the second node and ground. A first circuit is coupled to (1) the first and second nodes and (2) ground for detecting a phase difference between a first input signal and a second input signal. The first circuit has a first transistor coupled to the first node and a third node, a second transistor coupled to the second and third nodes, and a third transistor coupled to the third node and ground via a first current source. The first transistor receives the first input signal. The second transistor receives a reference signal. The third transistor receives the second input signal. The first and second signals have different voltage swing characteristics.

Other objects, features, and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not limited to the figures of the accompanying drawings, in which like references indicate similar elements and in which:

FIG. 1 illustrates a phase detector in accordance with one embodiment of the present invention;

FIG. 2 illustrates the waveform of the output signal of the phase detector of FIG. 1 in view of various input signals;

FIG. 3 is a circuit diagram of a phase detector in accordance with another embodiment of the present invention;

FIG. 4 illustrates the waveform of the output signal of the phase detector of FIG. 3 in view of various input signals.

DETAILED DESCRIPTION

FIG. 1 illustrates the circuit of a quadrature phase detector 10 that implements one embodiment of the present invention. Phase detector 10 of FIG. 1 detects the quadrature phase error between two input signals that are in quadrature phase relationship and that have different voltage swing characteristics. FIG. 3 illustrates the circuit of another quadrature phase detector 40 that implements another embodiment of the present invention. Phase detector 40 minimizes the phase detection error induced by the parasitic capacitance.

Phase detectors

10 and 40 will be described in more detail below.

Phase detector

10 shown in FIG. 1 includes transistors 11 through 14 and 19 through 21. For one embodiment, transistors 11-14 and 19-21 are metal oxide semiconductor field effect transistors (“MOSFETs”) and are in a complementary metal oxide semiconductor (“CMOS”) configuration. For other embodiments, transistors 11-14 and 19-21 can be N-channel MOSFET transistors or P-channel MOSFET transistors. For alternative embodiments, other devices, such as bipolar transistors, may be used.

For one embodiment shown in FIG. 1, transistors 11-14 are P-channel transistors that are connected as current sourcing transistors between a power supply voltage V_DDand

nodes

15 and 16. Alternatively, transistors 11-14 can be N-channel transistors or bipolar transistors.

Transistors 11-14 together constitute the load of phase detector 10. Transistors 11-14 present a high differential impedance between nodes 15-16 and a low common mode resistance from the power supply V_DDto nodes 15-16. Diode connected transistors 11-12 serve as a low common mode resistance between the power supply V_DDand nodes 15-16. Transistors 11-12 also constitute a positive differential load resistance between

nodes

15 and 16. Transistors 13-14 constitute a negative differential load resistance between

nodes

15 and 16. The negative differential load resistance cancels the positive differential load resistance. As a result, transistors 11-14 together present the high differential load resistance between

nodes

15 and 16. The connection of transistors 11-14 is described below.

Alternatively, other types of load circuits can be used in phase detector 10 between the power supply V_DDand nodes 15-16. In addition, the load formed by transistors 11-14 can be any other kind of high differential impedance load circuit.

The drain of transistors 11-14 are connected to node 15 and the drain of

transistors

12 and 13 are connected to node 16. Each of transistors 11-12 has its gate coupled to its drain. In addition, the gate of transistor 13 is connected to the gate of transistor 11, and the gate of transistor 14 is connected to the gate of transistor 12. Because the gates of

transistors

11 and 13 are connected together and the gates of

transistors

12 and 14 are connected together, transistor 13 mirrors the current through transistor 11 and transistor 14 mirrors the current through transistor 12. In other words,

transistors

11 and 13 constitute a current mirror and

transistors

12 and 14 constitute another current mirror. By mirroring the current flowing through transistor 11 to node 16 and by mirroring the current flowing through transistor 12 to node 15, each of nodes 15-16 receives substantially the same amount of current and no differential current is generated. This therefore causes transistors 11-14 to present a high differential load resistance because the negative differential load resistance generated by transistors 13-14 cancels the positive differential load resistance generated by transistors 11-12.

For one embodiment, transistors 11-14 are substantially identical in size such that the negative differential load resistance generated by transistors 13-14 cancels the positive differential load resistance generated by transistors 11-12.

Nodes 15-16 form the output of phase detector 10. A capacitor 17 is connected between ground and node 15 and a capacitor 18 is connected to node 16 and ground. For one embodiment,

capacitors

17 and 18 have substantially equivalent capacitance. For one embodiment,

capacitors

17 and 18 include parasitic capacitances of transistors 11-14 at nodes 15-16, respectively. Alternatively,

capacitors

17 and 18 may or may not comprise parasitic capacitances, such as those of transistors 11-14 at nodes 15-16, respectively.

Node

15 is further connected to the drain of transistor 19 and node 16 is further connected to the drain of transistor 20. The sources of transistors 19-20 are connected to a node 23. Node 23 is then connected to the drain of transistor 21. Transistor 21 has its source connected to ground via a current source 24. The gate of transistor 19 receives an input signal V_IN2. The gate of transistor 20 receives an input signal V_REF. The gate of transistor 21 receives an input signal V_IN1. Transistors 19-21 are N-channel transistors. Alternatively, transistors 19-21 can be P-channel transistors or bipolar transistors. For one embodiment, transistor 19 has a size that is substantially equal to that of transistor 20.

As seen from FIG. 2, the V_IN1signal exhibits a full CMOS voltage swing. The V_IN2signal, however, is a small voltage swing signal that oscillates substantially symmetrically around the V_REFreference voltage. As can be seen from FIG. 2, the V_REFsignal is a constant DC reference voltage. The V_IN2signal is therefore referred to as quasi-differential signal. In other words, the V_IN2and V_REFsignals are not complementary to each other. The V_IN1and V_IN2signals are shown in the quadrature phase relationship in FIG. 2.

Alternatively, the V_IN2signal is a small swing, fully differential signal that swings between a V_highvoltage and a V_lowvoltage. In this case, the V_REFsignal is complementary to the V_IN2signal. In other words, when the gate of transistor 19 receives the V_highvoltage, the gate of transistor 20 receives the V_lowvoltage.

Transistors 19-21 detect the quadrature phase error of the V_IN1and V_IN2input signals. It is desirable to have the V_IN1and V_IN2signals in a quadrature phase relationship. When quadrature phase error occurs (i.e., the desired quadrature phase relationship has not been achieved), phase detector 10 detects that condition by producing a net differential voltage across nodes 15-16 (i.e., the output V_OUT) at the end of each measurement cycle. The voltage level of the net differential voltage across nodes 15-16 is a function of the amount of quadrature phase error between the V_IN1and V_IN2input signals. If phase detector 10 does not detect any quadrature phase error, phase detector 10 does not produce any net differential voltage across nodes 15-16 at the end of the detection cycle.

Phase detector

10 also includes a transistor 22 coupled between nodes 15-16. Transistor 22 is an N-channel MOSFET transistor. Alternatively, transistor 22 can be a P-channel MOSFET transistor or a bipolar transistor. Transistor 22 is used in phase detector 10 as an equalizing transistor. Transistor 22 causes the voltage difference across nodes 15-16 to be zero when transistor 22 conducts before a measurement cycle is initiated. As can be seen from FIG. 1, transistor 22 is switched on or off by the V_EQsignal. When transistor 22 is turned on by the V_EQsignal,

nodes

15 and 16 are connected together via transistor 22 and the voltages at nodes 15-16 are equalized.

FIG. 2 also illustrates the signal waveform of the V_EQsignal. As can be seen from FIG. 2, the V_EQsignal can be a periodic pulse signal that occurs before every pulse of the V_IN1signal. The V_EQsignal helps to equalize the voltages across nodes 15-16 for starting a detection cycle. Alternatively, the pulse cycle of the V_EQsignal can occur before every Nth pulse of the V_IN1signal.

Referring to FIGS. 1-2, the operation of phase detector 10 is now described. As shown in FIG. 1, transistor 21 connects a current I from node 23 to current source 24 when the V_IN1signal is at the high V_DDvoltage. Because transistor 21 is connected to current source 24, transistor 21, when conducting, only allows the amount of current I to flow through. The V_IN1signal controls the start of each detection cycle. Whenever the voltage level of the V_IN1signal rises to the V_DDvoltage, a detection cycle is initiated.

When the voltage level of the V_IN2signal is higher than that of the V_REFvoltage and when the V_IN1signal is at the V_DDvoltage (e.g., from time t₁to time t₂), transistor 19 conducts more than transistor 20 does. Transistor 19 thus contributes substantially all the I current to node 23. Because each of

nodes

15 and 16 receives substantially the same amount of current from the load element formed by transistors 11-14, capacitor 17 is charged differently than capacitor 18 if

transistors

19 and 20 are not drawing the same amount of current to node 23. In this case, capacitor 18 may be charging while capacitor 17 is discharging. This in turn generates a differential voltage across nodes 15-16, and therefore at the output V_OUTof phase detector 10. The differential voltage at the output V_OUTramps linearly upwards for the time that the voltage of the V_IN2signal is higher than the V_REFreference voltage (e.g., from time t₁to time t₂in FIG. 2). The signal waveform of the V_OUTsignal is also shown in FIG. 2.

When the voltage level of the V_IN2signal is lower than that of the V_REFvoltage and when the V_IN1signal is at the V_DDvoltage (e.g., from time t₂to time t₃), transistor 20 then contributes substantially all the I current to node 23. This in turn causes the charging of capacitors 17-18 to be uneven. In this case, capacitor 17 may be charging while capacitor 18 is discharging. This then results in the differential voltage at the output V_OUTof phase detector 10 to drop linearly downwards (see the waveform of the V_OUTsignal in FIG. 2).

When the voltage level of the V_IN1signal goes to ground, the differential voltage at the output V_OUTstops changing. If the V_IN1and V_IN2signals are in perfect quadrature, the differential voltage across nodes 15-16 ramps linearly towards zero and no net differential voltage will be developed at the V_OUToutput of phase detector 10 at the time when the V_IN1signal goes to ground (e.g., at time t₃). If, however, a quadrature phase error exists between the V_IN1and V_IN2signals, a net differential voltage will be developed across nodes 15-16 at the end of the phase detection cycle (e.g., from time t₁to time t₃). The net differential voltage across nodes 15-16 is substantially proportional to the amount of the quadrature phase error.

The output V_OUTof phase detector 10 can be connected to a comparator to generate a binary quadrature phase error output. Other circuits may also be used to generate the binary quadrature phase error output.

The above description of phase detector 10 in detecting the quadrature phase error assumes the desirable situation in which the influence of any parasitic capacitance of transistors 19 through 21 at node 23 is neglected. As can be seen from FIG. 1, phase detector 10, however, includes a parasitic capacitor 25 coupled between node 23 and ground. Parasitic capacitor 25 includes the parasitic capacitance of transistors 19-21 at node 23, as well as other parasitic capacitances. Because transistor 21 is used as a switch, parasitic capacitor 25 includes the parasitic capacitance between the source of transistor 21 and ground.

Due to the existence of parasitic capacitor 25 in the circuit of phase detector 10, phase detector 10 generates a net differential voltage at the output V_OUTof the circuit at the end of a detection cycle even when the V_IN1and V_IN2signals are in the perfect quadrature phase relationship, as can be seen from FIG. 2 (e.g., at time t₃). The generation of the net differential voltage at the V_OUToutput of phase detector 10 by parasitic capacitor 25 is described below.

As can be seen from FIG. 1, due to parasitic capacitor 25, the voltage level at node 23 does not change immediately after transistor 21 is turned on by the V_IN1signal, causing the current flowing through transistor 21 to exceed the current I, which generates an additional error current. Because the voltage level of the V_IN2signal is higher than the V_REFvoltage, this additional error current flows through transistor 19. This in turn causes capacitor 17 to be additionally discharged by the additional error current, which results in the differential voltage at the output V_OUTto be ramped higher.

When, however, the voltage level of the V_IN2signal drops below the V_REFvoltage, transistor 19 is much less conducting than transistor 20, and transistor 20 contributes substantially all the I current to node 23. The voltage level at node 23 falls. This causes parasitic capacitor 25 to be discharged, reducing the current flowing through transistor 20, which generates an additional error current flowing through transistor 20 to capacitor 18. This additional error current causes capacitor 18 to be additionally charged.

In addition, due to parasitic capacitor 25, the voltage level at node 23 does not change immediately after transistor 21 is turned off by the V_IN1signal, causing an additional error current to flow through transistor 20 to parasitic capacitor 25. This additional error current causes capacitor 18 to be additionally discharged.

As a result, all the above-described error currents in charging capacitors 17-18 cause a net differential voltage at the output V_OUTof phase detector 10 at the end of a detection cycle even when the V_IN1and V_IN2signals are in the perfect quadrature phase relationship, as can be seen from FIG. 2. When this occurs, a phase detection error occurs. Because of the unpredictable nature of the parasitic capacitance, the voltage level of the net differential voltage due to the parasitic capacitance also cannot be predicted.

Phase detector

40 of FIG. 3 eliminates the net differential voltage occurring at the output of the circuit due to the parasitic capacitance.

As can be seen from FIG. 3, phase detector 40 includes transistors 41 through 44 connected between the power supply V_DDand

nodes

45 and 46. The connection and function of transistors 41-44 in phase detector 40 are identical to that of transistors 11-14 in phase detector 10 of FIG. 1, and therefore will not be described in more detail below.

Node

45 is connected to capacitor 47 and capacitor 48 is connected to node 46. The capacitance of capacitor 48 is substantially equal to that of capacitor 47. Nodes 45-46 are then connected to a first circuit formed by transistors 49 through 51 and a second circuit formed by transistors 52 through 54.

Transistors

51 and 54 are then connected to a circuit 60, which essentially includes a first current source for providing a first current I₁through transistors 51 and a second current source for providing a second current I₂through transistor 54, as can be seen from FIG. 3. Both the I₁and I₂currents are generated and controlled by a I_BIAScurrent.

Circuit

60 generates the I₁and I₂currents. The value of the I₂current is smaller than that of the I₁current. For one embodiment, the value of the I₂current is in a range of 20% to 30% of the I₁current. For alternative embodiments, the value of the I₂current can be larger or smaller than 20% to 30% of the I₁current.

For one embodiment, transistors 49-51 and 52-54 are N-channel MOSFET transistors. For alternative embodiments, transistors 49-51 and 52-54 can be P-channel MOSFET transistors or bipolar transistors. For one embodiment, the size of each of transistors 52-53 is substantially equal to that of each of transistors 49-50 and the size of transistor 54 is substantially equal to that of transistor 51.

Transistor

49 is connected to node 45 and node 55. Transistor 50 is connected to

nodes

46 and 55. Transistor 51 connects node 55 to ground via the current source I₁formed by circuit 60. Similarly, transistor 52 is connected to node 46 and a node 56. Transistor 53 is connected to

nodes

45 and 56. Transistor 54 connects node 56 to ground via the current source I₂formed by circuit 60. The gate of each of

transistors

51 and 54 receives the V_IN1signal. The gate of each of

transistors

49 and 52 receives the V_IN2signal and the gate of each of

transistors

50 and 53 receives the V_REFvoltage. The V_IN1and V_IN2signals and the V_REFvoltage are identical to those described above and shown in FIGS. 1-2. FIG. 4 also illustrates the signal waveforms of these signals.

FIG. 3 also illustrates a parasitic capacitor 57 connected to node 55 and ground and a parasitic capacitor 58 connected to node 56 and ground. Parasitic capacitor 57 includes the parasitic capacitance of transistors 49-51 at node 55 and parasitic capacitor 58 includes the parasitic capacitance of transistors 52-54 at node 56. Parasitic capacitor 57 also includes other parasitic capacitances. Because transistor 51 is used as a switch, parasitic capacitor 57 includes the parasitic capacitance between the source of transistor 51 and ground. Likewise, parasitic capacitor 58 also includes other parasitic capacitances. Because transistor 54 is used as a switch, parasitic capacitor 58 includes the parasitic capacitance between the source of transistor 54 and ground.

Transistors 49-51 detect the phase error of the V_IN1and V_IN2signals. Transistors 52-54 cancel the net differential voltage at the output V_OUTof phase detector 40 due to parasitic capacitor 57 in the circuit. As described above, each of transistors 52-53 has a size that is substantially equal to that of each of transistors 49-50. Therefore, the capacitance of parasitic capacitor 58 is substantially equal to that of parasitic capacitor 57. Due to the negative cancellation effect of transistors 52-54, the additional error currents generated in the circuit due to parasitic capacitors 57-58 cancel each other and phase detector 40 of FIG. 3 does not experience any net differential voltage generated at the output V_OUTof the circuit due to the parasitic capacitance. Because the capacitance of parasitic capacitors 57-58 is substantially equal, the additional error currents associated with parasitic capacitors 57-58 are also substantially equal. Transistors 49-50 and 52-53 are, however, cross-connected such that their respective contributions subtract from each other. This causes the additional error currents to cancel each other. This thus allows phase detector 40 to detect the quadrature phase error of the V_IN1and V_IN2signals with minimized phase detection error.

The operation of transistors 52-54 is described below, in conjunction with FIGS. 3 and 4. As can be seen from FIG. 3, when transistor 51 is turned on by the logical high V_IN1signal, the voltage level at node 55 does not change immediately, causing the current flowing through transistor 51 to exceed the current I₁, which generates an additional error current. Because the voltage level of the V_IN2signal is higher than the V_REFvoltage at this time, this additional error current flows through transistor 49, causing capacitor 47 to be additionally discharged. Meanwhile, because transistor 54 is also turned on by the logical high V_IN1signal, the voltage level at node 56 does not change immediately, which also generates an additional error current flowing through transistor 54. As the voltage level of the V_IN2signal is higher than the V_REFvoltage at this time, an additional error current flows through transistor 52, causing capacitor 48 to be additionally discharged. Because the capacitance of parasitic capacitor 57 is equal to that of parasitic capacitor 58, the additional error current that flows through transistor 52 is substantially equal to the additional error current through transistor 49. Given that transistor 52 is connected to node 46 while transistor 49 is connected to node 45, the additional error current generated by transistor 52 cancels that generated by transistor 49.

When the voltage level of the V_IN2signal is lower than that of the V_REFvoltage, transistor 49 is much less conducting than transistor 50 and transistor 52 is much less conducting than transistor 53. The voltage level at each of

nodes

55 and 56 falls. This causes

parasitic capacitors

57 and 58 to be discharged, reducing the current flowing through

transistors

50 and 53, respectively. This then causes an additional error current to flow through transistor 50 to capacitor 48 and an additional current to flow through transistor 53 to capacitor 47, additionally charging

capacitors

47 and 48, respectively. Given that transistor 50 is connected to node 46 and transistor 53 is connected to node 45 and given that the additional error currents through

transistors

50 and 53 are substantially equal to each other, the additional charging to each of

capacitors

47 and 48 cancels each other.

In addition, due to

parasitic capacitors

57 and 58, the voltage level at each of

nodes

55 and 56 does not change immediately after

transistors

51 and 54 are turned off by the V_IN1signal, causing an additional error current to flow through transistor 50 to parasitic capacitor 51 and an additional error current to flow through transistor 53 to parasitic capacitor 58. The additional error currents cause

capacitors

47 and 48 to be additional discharged, respectively. Because the capacitance of parasitic capacitor 57 is substantially equal to that of parasitic capacitor 58, the additional error current flowing through transistor 50 is substantially equal to the additional error current flowing through transistor 53. Given that transistor 50 is connected to node 46 and transistor 53 is connected to node 45, the additional error currents cancel each other. By doing so, no net differential voltage due to parasitic capacitor 58 will be developed at the output V_OUTand phase detector 40 detects the quadrature phase error of the V_IN1and V_IN2signals with minimized detection error. The signal waveform of the V_OUTsignal of phase detector 40 is shown in FIG. 4.

The V_OUToutput of phase detector 40 can also be connected to a comparator to generate a binary quadrature phase error output. Alternatively, other circuits can be used to generate the binary quadrature phase error output.

Phase detector

40 also includes an equalizing transistor 59 connected between nodes 45-46. Transistor 59 ensures that phase detector 40 initiates a phase detection cycle with a zero differential voltage at the output V_OUT, and is controlled by the V_EQsignal.

The generation of the V_EQsignal can be controlled by a clock signal. Therefore, various clocking protocols can be used to generate the V_EQsignal pulses. For example, to increase sensitivity in some applications, the measurement can take place over a number of clock cycles. In those cases, the V_EQpulse is generated such that transistor 59 is activated only once every N clock cycles, wherein N is the number of clock cycles during which the phase error measurement is allowed to integrate.

For one embodiment, N is equal to two, as shown in FIG. 4. For alternative embodiments, N can be larger than two.

Thus, the phase detector described above includes first and second circuits that are constructed in substantially the same way and receive the same input signals. The first and second circuits are, however, cross-coupled together such that the error current generated by the second circuit cancels that generated by the first circuit in order to allow the circuit to perform the designated function with minimized error.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

Claims

What is claimed is:

1. A method of phase detection, comprising:

sourcing a first pair of currents to first and second nodes respectively using a first current mirror, and sourcing a second pair of currents to the first and second nodes respectively using a second current mirror, wherein the first and second current mirrors are cross coupled; and

charging and discharging capacitors associated with the first and second nodes in response to first and second input signals, wherein a voltage differential across the first and second nodes after the charging and discharging indicates an amount of quadrature phase difference between the first and second input signals.

2. The method of claim 1, wherein:

said sourcing the first pair of currents includes mirroring a first current of the first pair of currents from a second current of the first pair of currents; and

said sourcing the second pair of currents includes mirroring a first current of the second pair of currents from a second current of the second pair of currents.

3. The method of claim 1, wherein said charging and discharging the capacitors associated with the first and second nodes includes:

coupling a current sink path to a third node in response to a first state of the first input signal;

activating a conductive path between the third node and the first node in response to the second input signal, wherein, when the first input signal assumes the first state and the conductive path is activated, a first capacitor of the capacitors is charged in a first direction; and

deactivating the conductive path between the third node and the first node in response to the second input signal, wherein, when the first input signal is in the first state and the conductive path is deactivated, the first capacitor is charged in a second direction.

4. The method of claim 3 wherein said charging and discharging the capacitors associated with the first and second nodes further includes:

charging a second capacitor of the capacitors in the second direction while the first capacitor is being charged in the first direction; and

charging the second capacitor in the first direction while the first capacitor is being charged in the second direction.

5. The method of claim 3 wherein a detection cycle to measure the amount of quadrature phase difference occurs when the first input signal is in the first state.

6. The method of claim 3, further comprising:

isolating the current sink path from the third node in response to a second state of the first input signal; and

equalizing voltages across the first and second nodes when the first input signal is in the second state.

7. The method of claim 1, further comprising comparing voltage levels across the first and second nodes after the charging and discharging to generate a binary signal that is representative of the amount of quadrature phase difference between the first and second input signals.

8. A method of detecting a phase error between first and second input signals, the method comprising:

sourcing a first current to a first node;

sourcing a second current to a second node, wherein the second current is mirrored from the first current;

sourcing a third to the second node;

sourcing a fourth current to the first node, wherein the fourth current is mirrored from the third current;

coupling a current sink path to a third node when a first input signal assumes a first state;

charging a capacitance associated with the first node in a first direction in response to a second input signal activating a conductive path between the third node and first node when the first input signal is in the first state; and

charging the capacitance associated with the first node in a second direction in response to the second input signal deactivating the conductive path between the third node and the first node when the first input signal is in the first state, wherein a voltage level present on the first node, after charging the capacitance in the second direction, indicates amount of phase difference between the first and second input signals.

9. The method of claim 8, wherein a phase of the first input signal relative to the second input signal approaches quadrature, and wherein the voltage level present on the first node indicates an amount of quadrature phase error between the first and second input signals.

10. The method of claim 8, further comprising:

charging a capacitance associated with the second node in the second direction when the capacitance associated with the first node is being charged in the first direction; and

charging the capacitance associated with the second node in the first direction when the capacitance associated with the first node is being charged in the second direction.

11. The method of claim 10, wherein a voltage level is present on the second node after charging the capacitance associated with the second node in the first direction.

12. The method of claim 11, further comprising comparing the voltage levels present on the first and second nodes to each other, to generate a binary signal that is representative of an amount of quadrature phase difference between the first and second input signals.

13. The method of claim 8, wherein a detection cycle to measure the amount of phase difference occurs when the first input signal is in the first state.

14. The method of claim 8, further comprising:

isolating the current sink path from the third node when the first input signal assumes a second state; and

15. A method of phase detection in a circuit that includes a load having first and second current mirrors cross coupled at respective first and second nodes, wherein the method comprises:

receiving a first input signal, wherein a transistor couples a current sink path to a third node in response to a first state of the first input signal;

activating a current path between the first node and the third node in response to a second input signal; and

producing a voltage level at the first node after activating the current path between the first node and the third node, wherein the voltage level indicates an amount of phase difference between the first and second input signals, wherein producing the voltage level includes:

charging a capacitance associated with the first node in a first direction when the current path between the first node and the third node is activated; and

charging the capacitance associated with the first node in a second direction when the current path between the first node and the third node is inactivated.

16. The method of claim 15, further comprising comparing the voltage produced at the first node to a voltage produced at the second node to generate a binary signal that is representative of the amount of phase difference between the first and second input signals.

17. The method of claim 15, wherein a phase of the first input signal relative to the second input signal approaches quadrature, and wherein the voltage level produced at the first node indicates an amount of quadrature phase error between the first and second input signals.

18. The method of claim 15, further comprising:

19. The method of claim 18 wherein a detection cycle to measure the amount of phase difference occurs when the first input signal is in the first state.

20. The method of claim 15, further comprising:

isolating the current sink path from the third node, using the transistor, in response to a second state of the first input signal; and

21. The method of claim 15, further including receiving a reference voltage.